Optical fiber

ABSTRACT

An improved optical fiber achieves both reduced bending and microbending losses, as well as a much higher Brillouin threshold, as compared to standard transmission fibers. The optical fiber comprises a core including at least two dopants and having a refractive index difference Δn 1  with an outer optical cladding, a first inner cladding having a refractive index difference Δn 2  with the outer cladding, and a depressed, second inner cladding having a refractive index difference Δn 3  with the outer cladding of less than −3×10 −3 . The radial concentration of at least one of the core dopants varies continuously over the entire core region of the optical fiber.

CROSS-REFERENCE TO PRIORITY APPLICATION

This application claims the benefit of pending European Application No.06291866.9 (filed Dec. 4, 2006, at the European Patent Office), which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to the field of optical fiber transmission and,more specifically, to an optical fiber having reduced losses due tostimulated Brillouin scattering (SBS) and reduced bending andmicrobending losses.

BACKGROUND OF THE INVENTION

A refractive index profile of an optical fiber is a graphicalrepresentation of the value of the refractive index as a function ofoptical fiber radius. Conventionally, the distance r to the center ofthe fiber is shown along the abscissa, and the difference between therefractive index and the refractive index of the fiber cladding is shownalong the ordinate axis. The optical fiber refractive index profile isreferred to as a “step” profile, a “trapezoidal” profile, a “parabolic”profile, or a “triangular” profile for graphs having the respectiveshapes of a step, a trapezoid, a parabola, or a triangle. These curvesare generally representative of the theoretical or reference indexprofile (i.e., set profile) of the fiber. The fiber manufacturingconstraints and stresses may lead to a slightly different profile.

An optical fiber typically includes an optical core, whose function isto transmit and possibly to amplify an optical signal, and an opticalcladding, whose function is to confine the optical signal within thecore. For this purpose, the refractive indexes of the core n_(c) and theouter cladding n_(g) are such that n_(c)>n_(g). As is well known, thepropagation of an optical signal in a single-mode optical fiber isdivided into a fundamental mode (i.e., dominant mode) guided in the coreand into secondary modes (i.e., cladding modes) guided over a certaindistance in the core-cladding assembly.

Optical fibers are key components in modern telecommunication systems.Operators are constantly concerned about increasing the optical powertransmitted along the fiber while limiting aging and losses of theoptical fiber. For logistical reasons, operators are concerned aboutreducing the number of different kinds of fibers and are willing to usethe same kind of fibers as feeder and termination fibers and as linefibers. Termination fibers need to have low bending sensitivity as theygenerally experience small bending radii in their installation. Feederfibers need to have reduced Brillouin scattering as they distribute highinput power into the telecommunication system.

One limitation for use of such optical fibers for telecommunicationapplications is loss due to stimulated Brillouin scattering (SBS). SBSis an optical nonlinearity due to interaction of optical photons withacoustic phonons of the glass matrix constituting the optical fiber. SBSlimits the maximum optical power throughput of the optical fibertransmission system; as input power increases above what is known as theBrillouin threshold, the power that can be transmitted along the opticalfiber reaches an upper limit. Any additional input power to the opticalfiber scatters in the backward direction because of the interaction withacoustic phonons rather than propagating in the forward, launchdirection as a higher power signal. Thus, SBS reduces thesignal-to-noise ratio at the receiver and can cause the transmitter tobecome unstable as a result of the entry of reflected light. Moreover,the increasing use of optical amplifiers and solid state Nd:YAG lasersat ever increasing data rates over longer and longer distances allcombine to exacerbate SBS.

Exemplary techniques suggested in the literature to increase theBrillouin threshold, minimize the detrimental effects of SBS, andincrease the power handling capacity of the optical fiber rely onbroadening either the photon energy spectrum of the source or the phononenergy spectrum of the glass to reduce the efficiency of theinteraction. A broadening of the spontaneous Brillouin spectrum widthwill increase the Brillouin threshold. This can be achieved by makingthe Brillouin frequency shift to vary in the fiber section or along thefiber length.

European Patent No. 0839770 (and its counterpart U.S. Pat. No.5,851,259) propose modulating drawing tension along the fiber tosuppress SBS with no significant change in fiber loss or dispersionfactors.

Japanese Patent Publication No. 09-311231 proposes changing therefractive index profile along the length of the fiber (i.e., axially)by varying the background fluorine concentration.

International Publication No. 2004/027941 proposes changing therefractive index profile along the length of the fiber by application ofultraviolet radiation or by thermal treatment.

Japanese Patent Publication No. 09-048629 discloses an optical fiberthat includes a core region in which germanium dopant decreases from acentral part to an outer periphery and fluorine dopant decreases fromthe outer periphery to the central part. The glass viscosity in thefiber cross section is therefore uniformly adjusted to prevent residualstress during fiber drawing.

Japanese Patent Publication No. 09-218319 discloses an optical fiberwith reduced Brillouin scattering. The core diameter varies in thelongitudinal direction of the optical fiber and includes a first dopantto increase refractive index and to lower the velocity of longitudinalacoustic waves and a second dopant to lower refractive index and tolower the longitudinal acoustic waves.

U.S. Patent Application Publication No. 2002/0118935 A1 proposes anirregular coating surrounding the optical cladding that varies in alengthwise direction in order to alter the mode profile of the acousticwaves.

“Stimulated Brillouin Scattering Suppression by Means of Applying StrainDistribution to Fiber with Cabling,” N. Yoshizawa et al., IEEE JLT, Vol.11, No. 10, pp. 1518-1522, (1993), proposes wrapping the fiber around acentral rod to induce stress to change the energy distribution ofacoustic phonons.

Some disadvantages of changing the index of refraction along the axialdirection of the fiber, and tight fiber wrapping, include non-uniformfiber properties (e.g., splicing characteristics, Raman gain, andcut-off wavelength) along the fiber length and increased fatigue, whichimpacts optical fiber life.

U.S. Pat. No. 6,542,683 proposes broadening the energy spectrum ofparticipating SBS phonons by providing a fiber core that includesalternating layers of glass-modifying dopant, which leads to non-uniformthermal expansion and viscosity profiles that impart a residualpermanent non-uniform stress in the fiber section. At least two layersof differing coefficients of thermal expansion (CTE) and viscositiesgenerate strain variation in the fiber section. This, in turn, generatesBrillouin frequency shift variation, and hence linewidth increase of themode.

Coefficients of thermal expansion and viscosity control in alternatinglayers are hard to achieve, and manufacturing processes capable ofobtaining a preform of doped and undoped layers within the core requirescostly equipment. Moreover, whenever the core is doped, fiber lossesincrease. This is especially so to the extent dopant concentrations havedistinct variations (e.g., step-change variation). Such sharp variationswill induce silica network defects at its interfaces, causing increasedabsorption loss of the fiber and degraded aging behavior.

U.S. Pat. No. 6,587,623 proposes controlling acoustic waves to be guidedaway from the portion of the waveguide that guides the light (i.e.,guiding acoustic waves into the cladding) to reduce photon-phononinteraction and thus reduce SBS. Such an optical fiber is difficult toachieve, however, as the optical fiber refractive index profile mustsimultaneously satisfy good light guiding and bad acoustic guiding. Intrying to reduce SBS in this way, drawbacks in optical transmissionproperties are expected.

“Effective Stimulated Brillouin Gain in Single Mode Optical Fibers,” J.Botineau et al., Electronics Letters, Vol. 31, No. 23, (Nov. 9, 1995),establishes that fibers possessing a trapezoidal refractive indexprofile achieve a higher Brillouin threshold compared to fiberspossessing a step refractive index profile. Trapezoid profile shapes,however, might not be well suited for certain telecommunicationapplications.

U.S. Publication No. 2004/0218882 A1 discloses an optical fiber having ahigh SBS threshold. The core includes three regions with a specificdoping scheme. The fiber refractive index profile disclosed in thisdocument might not be well suited for certain telecommunicationapplications.

For compatibility between the optical systems of differentmanufacturers, the International Telecommunication Union (ITU) hasestablished a standard referenced ITU-T G.652, which must be met by aStandard Single Mode Fiber (SSMF).

This G.652 standard for transmission fibers, recommends inter alia, arange of 8.6 microns to 9.5 microns for the Mode Field Diameter (MFD) ata wavelength of 1310 nanometers; a maximum of 1260 nanometers for thecabled cut-off wavelength; a range of 1300 nanometers to 1324 nanometersfor the dispersion cancellation wavelength (denoted λ₀); and a maximumchromatic dispersion slope of 0.092 ps/(nm²·km) (i.e., ps/nm²/km).

The cabled cut-off wavelength is conventionally measured as thewavelength at which the optical signal is no longer single mode afterpropagation over 22 meters of fiber, such as defined by subcommittee 86Aof the International Electrotechnical Commission under standard IEC60793-1-44.

Efforts to increase SBS threshold should not result in non-compliancewith the G.652 standard.

Moreover, high optical power in transmission optical fibers may damagethe fiber coating and thus accelerate aging of the optical fiberwherever bends are present. Reducing bending sensitivity of an opticalfiber having a high Brillouin threshold would reduce aging problems ofhigh power applications.

In addition, as previously noted, it is desirable to reduce bendingsensitivity of optical fibers for use as termination fibers.

Typical solutions to reduce bending losses are to influence the MACvalue. For a given fiber, the so-called MAC value is defined as theratio of the mode field diameter of the fiber at 1550 nanometers to theeffective cut-off wavelength λ_(ceff). The effective cut-off wavelengthis conventionally measured as the wavelength at which the optical signalis no longer single mode after propagation over two meters of fiber suchas defined by sub-committee 86A of the International ElectrotechnicalCommission under standard IEC 60793-1-44. The MAC value is used toassess fiber performance, particularly to achieve a compromise betweenmode field diameter, effective cut-off wavelength, and bending losses.

FIG. 1 depicts the experimental results that illustrate bending lossesat a wavelength of 1625 nanometers with a bend radius of 15 millimetersin a SSMF fiber in relation to the MAC value at a wavelength of 1550nanometers. FIG. 1 shows that the MAC value influences fiber bending andthat these bending losses may be reduced by lowering the MAC value.

That notwithstanding, a reduction in the MAC value by reducing the modefield diameter and/or by increasing the effective cut-off wavelength maylead to noncompliance with the G.652 standard, making the optical fibercommercially incompatible with some transmission systems.

Compliance with the G.652 standard while reducing bending losses andincreasing SBS threshold is a challenge for fiber applications in whichsingle access optical fibers are to be used both in long-haultransmission systems and in Fiber-to-the-Home (FTTH) orFiber-to-the-Curb (FTTC) systems.

“Bend-Insensitive and Low Splice-Loss Optical Fiber for Indoor Wiring inFTTH,” S. Matsuo et al., OFC 2004 Proceedings, Paper Th13 (2004),describes a refractive index profile for single mode fiber (SMF) thatpermits a reduction in bending losses. This disclosed fiber, however,shows a chromatic dispersion of between 10.2 ps/(nm·km) and 14.1ps/(nm·km), which lies outside the G.652 standard.

“Low Bending Loss and Low Splice Loss Single Mode Fibers Employing aTrench Profile,” S. Matsuo et al., IEICE Trans. Electron., Vol. E88-C,No 5 (May 2005), describes an optical fiber having a central core, afirst inner cladding, and a trench. Some of the exemplary fibersdescribed in this document meet the criteria of the G.652 standard.

Enhanced Bending Loss Insensitive Fiber and New Cables for CWDM AccessNetworks” I. Sakabe et al., 53^(rd) IWCS Proceedings, pp. 112-118(2004), proposes reducing the Mode Field Diameter to reduce bendinglosses. This reduction in mode field diameter, however, leads tooverstepping the G.652 standard.

“Development of Premise Optical Wiring Components Using Hole-AssistedFiber,” K. Bandou et al., 53^(rd) IWCS Proceedings, pp. 119-122 (2004),proposes a hole-assisted fiber having the optical characteristics of aSSMF fiber with reduced bending losses. The cost of manufacturing thisfiber and its high attenuation levels (>0.25 dB/km) reduce itscommercial viability in FTTH systems.

“Ultra-Low Loss and Bend Insensitive Pure-Silica-Core Fiber Complyingwith G.652 C/D and its Applications to a Loose Tube Cable,” T. Yokokawaet al., 53^(rd) IWCS Proceedings, pp. 150-155 (2004), proposes a puresilica core fiber (PSCF) having reduced transmission and bending losses,but with a reduced mode field diameter that falls outside the G.652standard.

U.S. Pat. No. 6,771,865 describes the refractive index profile of atransmission fiber with reduced bending losses. The optical fiber has acentral core, an annular inner cladding, and an optical outer cladding.The annular cladding is doped with germanium and fluorine. U.S. Pat. No.6,771,865 fails to disclose sufficient information to determine whetherits disclosed fiber meets the G.652 standard.

U.S. Pat. No. 4,852,968 describes the profile of a transmission fiberhaving reduced bending losses. This disclosed fiber, however, has achromatic dispersion that does not meet the G.652 criteria. The G.652standard requires cancellation of chromatic dispersion at wavelengths ofbetween 1300 nanometers and 1324 nanometers, but the fiber disclosed inU.S. Pat. No. 4,852,968 shows cancellation of chromatic dispersion atthe wavelengths of between 1400 nanometers and 1800 nanometers.

International Application No. 2004/092794 (and its counterpart U.S. Pat.No. 7,164,835) describe the refractive index profile of a transmissionfiber with reduced bending losses. The fiber has a central core, a firstinner cladding, a second depressed inner cladding, and an outer opticalcladding. Some of the exemplary fibers meet the criteria of the G.652standard. The disclosed fiber is manufactured by Vapor-phase AxialDeposition (VAD) or Chemical Vapor Deposition (CVD). InternationalApplication No. 2004/092794 fails to identify the problems ofmicrobending losses and Brillouin scattering.

In view of the foregoing, there is a need for a transmission fiber thatmeets the criteria of the G.652 standard (i.e., that can be usedcommercially in FTTH transmission systems of FTTH type) and that showsboth reduced bending and microbending losses and increased stimulatedBrillouin scattering threshold. Such optical fiber could be used as asingle access fiber (i.e., a line fiber for long-haul transmissionapplications and a feeder fiber or a termination fiber in FTTHapplications).

Furthermore, there is a need for an optical fiber having reduced bendinglosses and increased Brillouin threshold without unfavorably alteringits fiber transmission characteristics (e.g., with limited fiber lossincrease and without change to the fiber index profile).

SUMMARY OF THE INVENTION

Accordingly, in one aspect the invention embraces an optical fiber thatpossesses (i) a core having a radius r₁ that includes at least two coredopants, wherein the core has a refractive index difference Δn₁ with anouter optical cladding (e.g., an external optical cladding); (ii) afirst inner cladding (i.e., an intermediate cladding) having a radius r₂and a refractive index difference Δn₂ with the outer cladding; and (iii)a depressed, second inner cladding (i.e., a depressed trench) having aradius r₃ and a refractive index difference Δn₃ with the outer claddingof less than −3×10⁻³; and wherein the radial concentration of at leastone of the core dopants varies substantially continuously over the coreregion.

According to exemplary embodiments, the optical fiber of the presentinvention may include one or more of the following additional features:

-   -   The radial concentration of at least one of the core dopants        varies continuously over the entire core region;    -   The radial concentration of each of at least two core dopants        varies continuously over the entire core region;    -   The radial variation of at least one core dopant concentration        is such that its first derivative is proportional to the radial        power fraction P(r) of the optical signal transmitted in the        optical fiber;    -   The optical fiber has, at a wavelength of 1550 nanometers, a        spontaneous Brillouin spectrum width greater than or equal to        100 MHz;    -   The variation of at least one core dopant concentration        corresponds to an refractive index variation greater than or        equal to 1×10⁻³;    -   The core dopants are selected from germanium (Ge), fluorine (F),        phosphorus (P), aluminum (Al), chlorine (Cl), boron (B),        nitrogen (N), and/or alkali metals;    -   One of the core dopants is germanium (Ge), the germanium        concentration varying radially in the core between about 1 and        20 weight percent based upon the core's total composition (i.e.,        mass). In other words, at any position within the core, the        germanium concentration ranges between 1 and 20 weight percent        (i.e., a radial concentration of about 1-20 weight percent        germanium).    -   One of the core dopants is fluorine (F), the fluorine        concentration varying radially in the core between about 0.3 and        8 weight percent based upon the core's total composition (i.e.,        mass). In other words, at any position within the core, the        fluorine concentration ranges between 0.3 and 8 weight percent        (i.e., a radial concentration of about 0.3-8 weight percent        fluorine).    -   One of the core dopants is phosphorus (P), the phosphorus        concentration varying radially in the core between about 1 and        10 weight percent based upon the core's total composition (i.e.,        mass). In other words, at any position within the core, the        phosphorus concentration ranges between 1 and 10 weight percent        (i.e., a radial concentration of about 1-10 weight percent        phosphorus).    -   The depressed, second inner cladding includes germanium in a        radial concentration of between 0.5 weight percent and 7 weight        percent based upon the second inner cladding's total composition        (i.e., mass). Stated differently, at any position within the        second inner cladding, the germanium concentration is between        about 0.5 and 7 weight percent.    -   The refractive index difference Δn₃ between the second inner        cladding and the outer cladding is greater than about −15×10⁻³;    -   The optical fiber has, at a wavelength of 1550 nanometers, an        effective area greater than or equal to 50 μm²;    -   The optical fiber has, at a wavelength of 1550 nanometers, an        attenuation less than or equal to 0.3 dB/km;    -   The optical fiber has, at a wavelength of 1625 nanometers,        bending losses that are less than about 0.1 dB for a winding of        ten turns around a bend radius of 15 millimeters; less than        about 0.2 dB for a winding of one turn around a bend radius of        10 millimeters; and less than about 0.5 dB for a winding of one        turn around a bend radius of 7.5 millimeters;    -   The optical fiber has, at a wavelength of 1550 nanometers,        bending losses that are less than about 0.02 dB for a winding of        ten turns around a bend radius of 15 millimeters; less than        about 0.05 dB for a winding of one turn around a bend radius of        10 millimeters; and less than about 0.2 dB for a winding of one        turn around a bend radius of 7.5 millimeters;    -   The optical fiber has, up to a wavelength of 1625 nanometers,        microbending losses, measured by the so-called fixed diameter        drum method, of 0.8 dB/km or less.

In another aspect, the invention embraces an optical module or a storagebox that includes a housing receiving at least a wound portion of theoptical fiber according to the invention. According to exemplaryembodiment, the optical fiber is wound with a bending radius of lessthan about 15 millimeters (e.g., 10 millimeters or less) in the opticalmodule or the storage box according to the present invention.

In a related aspect, the invention embraces an optical system (e.g.,FTTH or FTTC) that includes at least one optical module or one storagebox according to the invention.

The foregoing, as well as other characteristics and advantages of thepresent invention, and the manner in which the same are accomplished,are further specified within the following detailed description and itsaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 (previously described) depicts bending losses at a wavelength of1625 nanometers with a bend radius of 15 millimeters in a standardsingle-mode fiber (SSMF) in relation to the MAC value at a wavelength of1550 nanometers.

FIG. 2 schematically depicts a cross-section (not to scale) of anexemplary optical fiber according to the present invention.

FIG. 3 depicts the nominal refractive index profile of an exemplarysingle-mode fiber according to the present invention.

FIG. 4 a depicts a reference refractive index profile of an exemplaryoptical fiber according to the present invention.

FIG. 4 b depicts germanium dopant concentration in the exemplary opticalfiber of FIG. 4 a.

FIG. 4 c depicts fluorine dopant concentration in the exemplary opticalfiber of FIG. 4 a.

FIG. 5 depicts dispersion characteristics for four different kinds ofoptical fibers.

DETAILED DESCRIPTION

In one aspect (and with reference to FIG. 2), the present inventionembraces an optical fiber 10 that includes a core 11 (i.e., the centralcore region in which the optical signal to be transmitted is guided) anda cladding region for confining the optical signal in the core 11. Thecladding region includes a first inner cladding 12, a depressed trench13 (or depressed, second inner cladding), and an outer cladding 14(e.g., an external optical cladding). The depressed trench 13 typicallyhas a refractive index difference with the outer cladding 14 that isless than −3×10⁻³ (e.g., less than about −15×10⁻³).

According to the present invention, the core region of the optical fiberincludes at least two dopants, the concentrations of which, in effect,vary continuously over the entire radius of the core region. Typically,the substantially continuous variation in radial dopant concentration isprogressive (e.g., increasing continuously in a radial direction) orregressive (e.g., decreasing continuously in a radial direction). Thatsaid, it is within the scope of the invention for the radial dopantconcentration to both increase and decrease in a radial direction.

According to the present invention, the variation of the first dopant(e.g., germanium) is compensated by variation of the second dopant(e.g., fluorine) to obtain a predetermined refractive index profile ofthe core region. The core region remains longitudinally homogeneousalong the optical fiber (i.e., concentrations of the core dopants areconstant along the optical fiber's length).

As will be appreciated by those having ordinary skill in the art,depending on the application, the optical fiber has a target refractiveindex profile that is defined according to various parameters (i.e.,mode field diameter, chromatic dispersion parameters, effective cut-offwavelength, and effective area).

Variation of dopant concentration in the optical fiber's radialdirection, particularly in its core, broadens the Brillouin spectrum andthereby increases the Brillouin threshold. A smooth dopant variationensures uniform mode power distribution for the different dopantconcentrations and limits fiber losses. Use of at least two dopants inthe optical fiber facilitates the achievement of a target refractiveindex profile and reduces the impact of SBS reduction on other opticalparameters, particularly mode field diameter and chromatic dispersionparameters. The optical fiber according to the present inventionpossesses a refractive index profile that meets the aforementioned G.652standard.

FIG. 3 depicts a nominal refractive index profile of a single-modetransmission optical fiber according to the present invention. Asschematically depicted in FIG. 2, the exemplary optical fiber 10includes (i) a central core 11 having refractive index difference Δn₁with an outer cladding 14; (ii) a first inner cladding 12 (i.e., anintermediate cladding) having a refractive index difference Δn₂ with theouter cladding 14; and (iii) a depressed trench 13 having an arefractive index difference Δn₃ with the outer cladding 14. The width ofthe core 11 is defined by its radius r₁ and the widths of the claddingsby their respective outer radii r₂ and r₃.

To define a nominal refractive index profile for an optical fiber, theindex of the outer cladding is generally taken as a reference. The indexvalues of the central core and of the claddings are then provided asindex differences (i.e., Δn_(1, 2, 3)) with the outer cladding.Generally, the outer cladding is formed of silica, but may be doped toincrease or reduce its refractive index, such as to modify the signalpropagation characteristics.

Each section of the refractive index profile of the optical fiber cantherefore be defined using integrals that associate the variations inrefractive indexes with the radius of each fiber section. See FIG. 2.

Three integrals thus can be defined for the optical fiber, whichrepresent the core surface I₁, the surface of the first inner claddingI₂ and the surface of the depressed, second inner cladding I₃. In thisregard, the expression “surface” is not to be interpreted geometrically(i.e., structurally) but should be understood instead to describe thearea under the curve (i.e., r·Δn) such as depicted in FIG. 3.

These three integrals can be expressed as follows:

I₁ = ∫₀^(r 1)Δ n(r) ⋅ r ≈ r₁ × Δ n 1I₂ = ∫_(r 1)^(r 2)Δ n(r) ⋅ r ≈ (r₂ − r₁) × Δ n 2I₃ = ∫_(r 2)^(r 3)Δ n(r) ⋅ r ≈ (r₃ − r₂) × Δ n 3

Table 1 (below) gives the limit values of radii and refractive indexdifferences, and the limit values of the integral I₁ that are requiredso that the optical fiber shows reduced bending losses and microbendinglosses while meeting the optical propagation criteria of standard G.652for transmission fibers. The values provided in the table are thenominal profiles of optical fibers according to the present invention.

TABLE 1 r₁ r₂ r₃ Δn₁ Δn₂ Δn₃ Δn₁ − Δn₂ I₁ (μm) (μm) (μm) r₁/r₂ (10⁻³)(10⁻³) (10⁻³) (10⁻³) (μm · 10⁻³) Min 3.5 7.5 12.0 0.27 4.2 −1.2 −15 3.917 Max 4.5 14.5 25.0 0.5 6.2 1.2 −3 5.9 24

The integral I₁ of the central core influences the shape and size of thefundamental propagation mode of the signal in the optical fiber. Anintegral value for the central core of between 17×10⁻³ microns and24×10⁻³ microns makes it possible in particular to maintain a mode fielddiameter that is compatible with the G.652 standard. In addition, thedepressed trench Δn₃ makes it possible to improve bending losses andmicrobending losses in SSMF.

According to the invention, the core region of the optical fiberincludes at least two dopants whose respective concentrations varysubstantially continuously over essentially the entire core region whilemaintaining the core region's pre-determined refractive index profile.Those having ordinary skill in the art will appreciate that radialdopant concentration might be unchanged over small increments (i.e.,radial segments) without departing from the scope of the invention. Thatsaid, as a practical matter, radial dopant concentration typicallyvaries continuously over the core radius. See FIGS. 4 b-4 c.

As noted, this allows broadening the Brillouin spectrum and therebyincreases the Brillouin threshold. Because the dopant concentrationvariation is compensated so as to keep a pre-determined refractive indexprofile, notably in the core region, the optical propagation criteria ofthe G.652 standard are not jeopardized by the presence of at least twodopants in the core. Moreover, the first inner cladding (Δn₂, r₂)ensures that the optical power remains in the core region without thedepressed trench (Δn₃, r₃) adversely impacting optical power throughput.

For a signal propagating at a wavelength of 1550 nanometers, the opticalfiber of the invention has a spontaneous Brillouin spectrum width thatis at least about 100 MHz. Such a broadened Brillouin spectrumeffectively increases the Brillouin threshold by at least a factor oftwo (or by about three dB in logarithmic scale) as compared to astandard single mode fiber (SSMF). The optical fiber of the inventionachieves a much higher Brillouin threshold compared to standardtransmission fibers with limited fiber loss (e.g., less than 0.3 dB/kmat a wavelength of 1550 nanometers) without significant change to theoptical transmission parameters.

The first core dopant (e.g., germanium) is chosen to achieve strong andcontinuous variations in density and elasticity in the fiber material.According to one embodiment, the radial distribution of the first dopantconcentration C_(d)(r) is such that its first derivative is proportionalto the radial power fraction P(r) of the optical signal transmitted inthe fiber in accordance with the following equation (in which α is aconstant):

$\frac{{C_{d}(r)}}{r} = {\alpha \cdot {P(r)}}$

This radial power fraction P(r) is expressed in watt per meters, theintegral of which is equal to the total transmitted power P according tothe following relationship:

∫P(r)dr=P

According to another embodiment, the depressed trench (i.e., the secondinner cladding) can include germanium at a concentration of betweenabout 0.5 and 7 weight percent, typically in a concentration of lessthan about 1.5 weight percent (e.g., between about 0.5 and 1.5 weightpercent), even if the index needs to be less than −3×10⁻³. The presenceof germanium in the depressed trench modifies the viscosity of silicaand the depressed trench's elasto-optical coefficient, thereby improvingmicrobending sensitivity.

FIGS. 4 a, 4 b, and 4 c relate to an exemplary optical fiber accordingto the present invention. The optical fiber of FIGS. 4 a-4 c possesses astep-core profile. The core has a given constant refractive index value;the depressed trench is separated from the core by an intermediate innercladding (i.e., the first inner cladding). FIG. 4 a illustrates theexemplary optical fiber's refractive index profile using arbitraryunits.

Turning to FIG. 4 b and FIG. 4 c, the core region of the fiber includesa first dopant, germanium (Ge), which is known to increase the value ofthe refractive index of silica, and a second dopant, fluorine (F), whichis known to decrease the value of the refractive index of silica. FIG. 4b and FIG. 4 c illustrate dopant concentrations in weight percent.According to the invention, the concentration of at least one of thecore dopants varies essentially continuously over the entire coreregion.

With respect to the exemplary optical fiber depicted in FIGS. 4 a, 4 b,and 4 c, both dopants vary continuously (and progressively) over theentire core region. The use of at least two dopants ensures that thecore refractive index profile is maintained to a nominal profile such toachieve desirable optical transmission characteristics. Indeed, becausethe second dopant can compensate for the refractive index variationintroduced by the variation of concentration of the first dopant, atarget refractive index profile can be achieved.

The variation of at least one core dopant concentration introducesdensity and elasticity variation in the optical fiber section thatbroadens the Brillouin spectrum and thereby increases the Brillouinthreshold. The variation of core dopant concentration should be largeenough to introduce sufficient density and elasticity variation in orderto reduce SBS.

The inventors have achieved satisfactory results if at least one of thecore dopants has a concentration variation over the entire core regionthat corresponds to a refractive index variation that is at least about1×10⁻³ (i.e., variation in core dopant concentration sufficient toachieve this refractive index variation if not compensated by anothercore dopant). In other words, the variation in first dopantconcentration (i.e., between the maximum and minimum radial dopantconcentrations) should be such that, without compensation by a seconddopant, a refractive index variation of at least about 1×10⁻³ would beachieved in the core.

As schematically depicted in FIGS. 4 b-4 c, the germanium concentrationvaries progressively from 5.8 weight percent to 12 weight percent, andthe fluorine concentration varies progressively from 0.1 weight percentto 1.7 weight percent.

The smooth and regular variation of dopant concentration ensures uniformmode power distribution for the different dopant concentration andlimits fiber losses. Simulations performed on an optical fiberexemplified in FIGS. 4 a, 4 b, and 4 c gives, at a signal wavelength of1550 nanometers, a spontaneous Brillouin spectrum width larger than 100MHz and an increased SBS threshold power (i.e., increased by at least afactor of two compared as compared to a standard single mode fiber), anda limited Rayleigh loss increase of about 0.013 dB/km. Despite thisRayleigh loss increase, the optical fiber of the present inventionmaintains compliance with the G.652 standard, having attenuation lossesof less than about 0.3 dB/km at 1550 nanometers.

As noted, FIGS. 4 a, 4 b, and 4 c represent one example of the presentinvention. Dopants other than germanium (Ge) and fluorine (F) can beused to achieve an optical fiber with reduced SBS according to thepresent invention. In this regard, the core region includes at least twodopants that may be selected from germanium (Ge), fluorine (F),phosphorus (P), aluminum (Al), chlorine (Cl), boron (B), nitrogen (N),and/or alkali metals. To the extent one of the core dopants is germanium(Ge), the concentration typically falls between about one and 20 weightpercent; to the extent one of the core dopants is fluorine (F), theconcentration is typically less than ten weight percent (e.g., betweenabout 0.3 and eight weight percent); to the extent one of the coredopants is phosphorus (P) the concentration typically falls betweenabout one and ten weight percent.

The exemplary optical fiber represented in FIGS. 4 a-4 c possesses adepressed trench to reduce its sensitivity to bending losses. Theoptical fiber of the invention therefore combines low bending losses andhigh Brillouin threshold.

Conventionally, starting from the teaching of J. Botineau et al. in“Effective Stimulated Brillouin Gain in Single Mode Optical Fibers,”Electronics Letters, Vol. 31, No. 23 (1995), one having ordinary skillin the art would choose a refractive index profile with triangular shapeor parabolic shape to increase the Brillouin threshold and might applyan external trench to reduce bending losses. This conventional approach,however, renders compliance with the G.652 specifications difficult.

FIG. 5 compares four different kinds of refractive index profile shapes:a typical step refractive index profile without trench (i.e., SSMF); astep-core refractive index profile with a trench in the cladding (e.g.,the optical fiber of FIGS. 4 a-4 c); a triangular core refractive indexprofile with a trench in the cladding; and a parabolic core profile witha trench in the cladding. For each, numerous refractive index profileswith different core diameter and maximum dopant level were simulated.

FIG. 5 depicts the zero dispersion wavelength λ₀ and the slope of thedispersion at the zero dispersion wavelength. The rectangle areaindicates the parameters of the G.652 specifications for those opticalcharacteristics. Fiber profiles having too high cut-off wavelengths andnon-conforming nominal mode-field diameter at 1310 nanometers so as tobe non-compliant with the G.652 specifications were omitted.

FIG. 5 shows that adding a depressed trench to a SSMF profile restrictsthe profile flexibility for production and thus increases fiberrejection rate. Using a triangular core refractive index profile with atrench in the cladding results in optical fibers that do not meet theG.652 requirements. A parabolic core refractive index profile with atrench in the cladding does yields some optical fibers within the G.652specifications, but the zone of tolerance is narrow and many rejectswould be expected.

The optical fiber of the invention achieves reduced bending andmicrobending losses, as well as a much higher Brillouin threshold,compared to standard transmission optical fibers. The optical fiber ofthe invention may be used in a receiver module of a FTTH system or in atransmitter module to input high power signals into a telecommunicationsystem, or in a high bit-rate long-haul optical transmission cable, withreduced optical losses. Moreover, the optical fiber according to thepresent invention is compatible with marketed systems as it meetsstandard G.652.

In one embodiment, the optical fiber of the present invention exhibits,at a wavelength of 1310 nanometers, a chromatic dispersion slope of0.092 ps/(nm²·km) or less; a cancellation of chromatic dispersion at awavelength of between 1300 and 1324 nanometers; and a cabled cut-offwavelength of 1260 nanometers or less.

In another embodiment, the optical fiber of the present invention has,at a wavelength of 1550 nanometers, an effective area superior or equalto 50 μm², typically 80 μm², and attenuation at 1550 nanometers of lessthan or equal to 0.3 dB/km. Such optical fiber according to thisembodiment is suitable for use in data transmission in telecommunicationsystems.

In this regard, an exemplary optical transmission system may include anoptical transmitter emitting optical signals in a predetermined range ofwavelength, a transmission optical fiber according to the presentinvention, and an optical receiver receiving the optical signal withimproved signal-to-noise ratio (SNR) due to reduced SBS and limitedincrease in fiber losses (e.g., attenuation). As compared toconventional systems, the optical transmitter may input into the opticalfiber an optical signal with higher power, the Brillouin threshold powerfor the transmission optical fiber being increased by at least a factorof two compared with a conventional SMF.

In yet another embodiment, the optical fiber of the present inventionhas, at a wavelength of 1625 nanometers, improved bending losses asfollows: less than about 0.1 dB for a winding of ten turns around a bendradius of 15 millimeters; less than about 0.2 dB for a winding of oneturn around a bend radius of ten millimeters; and less than about 0.5 dBfor a winding of one turn around a bend radius of 7.5 millimeters.

Likewise, the optical fiber of the present invention has, at awavelength of 1550 nanometers, improved bending losses as follows: lessthan about 0.02 dB for a winding of ten turns around a bend radius of 15millimeters; less than about 0.05 dB for a winding of one turn around abend radius of ten millimeters; and less than about 0.2 dB for a windingof one turn around a bend radius of 7.5 millimeters.

Moreover, for wavelengths of up to 1625 nanometers, the optical fiber ofthe present invention demonstrates microbending losses of less thanabout 0.8 dB/km measured by the so-called fixed diameter drum method.Accordingly, such optical fiber is suitable for implementation inoptical modules or storage boxes for use in FTTH or FTTC systems.Commonly assigned U.S. Patent Application Publication No. 2007/0258686A1 (and its corresponding U.S. application Ser. No. 11/743,365), each ofwhich is hereby incorporated by reference in its entirety, disclose thatthe fixed drum method is described in the technical recommendations bySubcommittee 86A of the International Electrotechnical Commission underreference IEC TR-62221.

In the specification and figures, typical embodiments of the inventionhave been disclosed. The present invention is not limited to suchexemplary embodiments. Unless otherwise noted, specific terms have beenused in a generic and descriptive sense and not for purposes oflimitation.

1. An optical fiber, comprising: a core surrounded by an outer cladding,said core (i) including at least two core dopants, wherein the radialconcentration of at least one of said core dopants varies continuouslyover said core, and (ii) having a radius r₁ and a refractive indexdifference Δn₁ with said outer cladding; a first inner claddingpositioned between said core and said outer cladding, said first innercladding having a radius r₂ and a refractive index difference Δn₂ withsaid outer cladding; and a depressed, second inner cladding positionedbetween said first inner cladding and said outer cladding, said secondinner cladding having a radius r₃ and a refractive index difference Δn₃with said outer cladding of less than −3×10⁻³.
 2. An optical fiberaccording to claim 1, wherein the radial concentration of each of saidat least two core dopants varies continuously over said core.
 3. Anoptical fiber according to claim 1, wherein the radial variation of atleast one core dopant concentration is such that its first derivative isproportional to the radial power fraction P(r) of an optical signaltransmitted in the optical fiber.
 4. An optical fiber according to claim1, wherein the optical fiber has, at a wavelength of 1550 nanometers, aspontaneous Brillouin spectrum width of at least about 100 MHz.
 5. Anoptical fiber according to claim 1, wherein the variation of at leastone core dopant concentration corresponds to a refractive indexvariation greater than about 1×10⁻³.
 6. An optical fiber according toclaim 1, wherein said at least two core dopants include one or more ofgermanium (Ge), fluorine (F), phosphorus (P), aluminum (Al), chlorine(Cl), boron (B), nitrogen (N), and/or alkali metals.
 7. An optical fiberaccording to claim 1, wherein one of said core dopants comprisesgermanium (Ge), the germanium concentration within said core varyingradially between about 1 and 20 weight percent.
 8. An optical fiberaccording to claim 1, wherein one of said core dopants comprisesfluorine (F), the fluorine concentration within said core varyingradially between about 0.3 and 8 weight percent.
 9. An optical fiberaccording to claim 1, wherein one of said core dopants comprisesphosphorus (P), the phosphorus concentration within said core varyingradially between about 1 and 10 weight percent.
 10. An optical fiberaccording to claim 1, wherein said depressed, second inner claddingcomprises germanium in a concentration between about 0.5 and 7 weightpercent.
 11. An optical fiber according to claim 1, wherein the indexdifference Δn₃ of said second inner cladding with said outer cladding isgreater than −15×10⁻³.
 12. An optical fiber according to claim 1, theoptical fiber having, at a wavelength of 1550 nanometers, an effectivearea greater than about 50 μm².
 13. An optical fiber according to claim1, the optical fiber having, at a wavelength of 1550 nanometers,attenuation less than about 0.3 dB/km.
 14. An optical fiber according toclaim 1, the optical fiber having, at a wavelength of 1625 nanometers,bending losses that are less than about 0.1 dB/m for a winding of tenturns around a bend radius of 15 millimeters.
 15. An optical fiberaccording to claim 1, the optical fiber having, at a wavelength of 1625nanometers, bending losses that are less than about 0.2 dB/m for awinding of one turn around a bend radius of 10 millimeters.
 16. Anoptical fiber according to claim 1, the optical fiber having, at awavelength of 1625 nanometers, bending losses that are less than about0.5 dB/m for a winding of one turn around a bend radius of 7.5millimeters.
 17. An optical fiber according to claim 1, the opticalfiber having, at a wavelength of 1550 nanometers, bending losses thatare less than about 0.02 dB/m for a winding of ten turns around a bendradius of 15 millimeters.
 18. An optical fiber according to claim 1, theoptical fiber having, at a wavelength of 1550 nanometers, bending lossesthat are less than about 0.05 dB/m for a winding of one turn around abend radius of 10 millimeters.
 19. An optical fiber according to claim1, the optical fiber having, at a wavelength of 1550 nanometers, bendinglosses that are less than about 0.2 dB/m for a winding of one turnaround a bend radius of 7.5 millimeters.
 20. An optical fiber accordingto claim 1, the optical fiber having, at a wavelength of up to 1625nanometers, microbending losses of about 0.8 dB/km or less as measuredby the fixed diameter drum method.
 21. An optical fiber according toclaim 1, wherein the refractive index difference Δn₁ between said coreand said outer cladding (i) is positive, and (ii) is greater than therefractive index difference Δn₂ between said first inner cladding andsaid outer cladding such that Δn₁>Δn₂.
 22. An optical fiber according toclaim 21, wherein r₁/r₂, the ratio of the radius of said core to theradius of said first inner cladding, is between about 0.27 and 0.5. 23.An optical fiber according to claim 21, wherein the value of thefollowing integral I₁ is between about 17×10⁻³ micron and 24×10⁻³micron: I₁ = ∫₀^(r 1)Δ n(r) ⋅ r ≈ r₁ × Δ n 1.
 24. A storagebox or an optical module comprising a housing, said storage box or saidoptical module receiving the optical fiber according to claim 1, whereinat least a portion of the optical fiber is wound with a bending radiusof less than 15 millimeters.
 25. A Fiber-to-the-Home (FTTH) opticalsystem or a Fiber-to-the-Curb (FTTC) optical system comprising at leastone optical module or one storage box according to claim
 24. 26. Anoptical fiber, comprising: a central core surrounded by an outer opticalcladding, said central core (i) including at least two core dopants,wherein the radial concentration of at least one of said core dopantsvaries substantially continuously over said central core, and (ii)having a radius r₁ that is between about 3.5 microns and 4.5 microns anda positive refractive index difference Δn₁ with said outer opticalcladding of between about 4.2×10⁻³ and 6.2×10⁻³; a first inner claddingpositioned between said central core and said outer optical cladding,said first inner cladding having a radius r₂ that is between about 7.5microns and 14.5 microns and a refractive index difference Δn₂ with saidouter optical cladding of between about −1.2×10⁻³ and 1.2×10⁻³; and adepressed, second inner cladding positioned between said first innercladding and said outer optical cladding, said second inner claddinghaving a radius r₃ that is between about 12.0 microns and 25.0 micronsand a refractive index difference Δn₃ with said outer optical claddingof between about −15×10⁻³ and −3×10⁻³.
 27. An optical fiber according toclaim 26, wherein the radial concentration of each of said at least twocore dopants varies substantially continuously over said central core.28. An optical fiber according to claim 26, wherein the optical fiberhas, at a wavelength of 1550 nanometers, a spontaneous Brillouinspectrum width of at least about 100 MHz.
 29. An optical fiber accordingto claim 26, wherein said at least two core dopants include one or moreof germanium (Ge), fluorine (F), phosphorus (P), aluminum (Al), chlorine(Cl), boron (B), nitrogen (N), and/or alkali metals.
 30. An opticalfiber according to claim 26, wherein a first said core dopant comprisesgermanium (Ge), the germanium concentration within said central corevarying radially between about 1 and 20 weight percent, and a secondsaid core dopant comprises fluorine (F), the fluorine concentrationwithin said central core varying radially between about 0.1 and 10weight percent.
 31. An optical fiber according to claim 26 the opticalfiber having, at a wavelength of 1625 nanometers, bending losses that(i) are less than about 0.1 dB/m for a winding of ten turns around abend radius of 15 millimeters, and (ii) are less than about 0.5 dB/m fora winding of one turn around a bend radius of 7.5 millimeters.
 32. Anoptical fiber according to claim 26, the optical fiber having, at awavelength of 1550 nanometers, bending losses that (i) are less thanabout 0.02 dB/m for a winding of ten turns around a bend radius of 15millimeters, and (ii) less than about 0.05 dB/m for a winding of oneturn around a bend radius of 10 millimeters.